Synthesis and complexation of a new tripodal tetradentate ligand, a silyl ligand tethered with three thioether moieties

Article abstract of DOI:10.1021/om100255p

Iridium(III) and platinum(II) complexes with a new tripodal tetradentate ligand, tris[2-(tert-butylthiomethyl)-phenyl]silyl were synthesized. In addition, reaction of tris-(2-((tert-butylthio)methyl)phenyl)silane with [PdCl₂(PhCN)₂]

Full text of DOI:10.1021/om100255p

Organometallics 2010, 29, 2839–2841 2839
DOI: 10.1021/om100255p
Synthesis and Complexation of a New Tripodal Tetradentate Ligand,
a Silyl Ligand Tethered with Three Thioether Moieties
Nobuhiro Takeda,* Daisuke Watanabe, Tetsu Nakamura, and Masafumi Unno*
Department of Chemistry and Chemical Biology and International Education and Research Center for
Silicon Science, Graduate School of Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu,
Gunma 376-8515, Japan
Received April 1, 2010
Summary: Iridium(III) and platinum(II) complexes with a
new tripodal tetradentate ligand, tris[2-(tert-butylthiomethyl)-
phenyl]silyl were synthesized. In addition, reaction of tris-
(2-((tert-butylthio)methyl)phenyl)silane with [PdCl2(PhCN)2]
resulted in the cleavage of the Si-C bond to give [{Pd[2-(t-
BuSCH2)C6H4]}2(μ-Cl)2].
transition-metal complexes with such ligands, it is expected
that the strong trans effect of the silyl moiety results in the
ready dissociation of the ligand on its trans position and thus
thioether moieties, weak σ-donors, can be readily exchanged
with other ligands. Such complexes are very interesting, due
to not only their unique structure and reactivity but also their
activity as catalysts and their ability to activate small mole-
cules. In this paper, we present the synthesis of a new tripodal
tetradentate silyl ligand, tris[2-(tert-butylthiomethyl)phenyl]-
silane -Si[2-(t-BuSCH2)C6H4]3 (1; Chart 1), and its coordination
chemistry with iridium(III), palladium(II), and platinum(II).
Transition-metal complexes bearing tripodal tetradentate
ligands have attracted considerable interest, due to their
utility in the activation of small molecules, the stabilization
of reactive species with unusual electric and geometric struc-
tures, catalytic activities, and so on.1 Although there have
been many reports on tripodal tetradentate ligands contain-
ing amine or phosphine moieties as donors, tripodal tetra-
dentate silyl ligands tethered with three donor moieties are
very rare.2 Stobart and co-workers reported the synthesis of
Chart 1
3
two types of tris(phosphino)silyl ligands, -Si(CH2CH2PPh2)3
and -Si[2-(Ph2P)C6H4CH2]3,4 and their complexation with
rhodium(III) and iridium(III), respectively. Recently, Peters
and co-workers reported the synthesis and properties of
dinitrogen complexes with Fe, Co, and Ir supported by
-Si[2-(R2P)C6H4]3 ligands.5 On the other hand, to the best
of our knowledge, tripodal tetradentate silyl ligands tethered
with three thioether moieties have not been reported. In
Synthesis of the precursor tris(2-((tert-butylthio)methyl)-
phenyl)silane, [2-(t-BuSCH2)C6H4]3SiH (3), is shown in
Scheme 1. The reaction of 1-bromo-2-(bromomethyl)benzene
with 2-methyl-2-propanethiol and NaH yielded 1-bromo-2-
(tert-butylthiomethyl)benzene (2) in good yield.6 Lithiation of
sulfide 2 with n-BuLi, followed by treatment with SiHCl3,
resulted in the formation of the silane precursor 3.
*To whom correspondence should be addressed. N.T.: tel, þ81-277-
30-1232; fax, þ81-277-30-1232; e-mail, ntakeda@chem-bio.gunma-u.
ac.jp. M.U.: tel:, þ81-277-30-1230; fax, þ81-277-30-1236; e-mail, unno@
chem-bio.gunma-u.ac.jp.
(1) (a) Hu, X. L.; Meyer, K. J. Organomet. Chem. 2005, 690, 5474–
5484. (b) Borovik, A. S. Acc. Chem. Res. 2005, 38, 54–61. (c) Schrock, R. R.
Acc. Chem. Res. 1997, 30, 9–16. (d) Huttner, G.; Strittmatter, J.; Sandhofner,
Scheme 1. Synthesis of Silane Precursor 3
€
S. In Comprehensive Coordination Chemistry II; Lever, A. B. P., Ed.;
Elsevier Pergamon: Oxford, U.K., 2004; Vol. 1, pp 297-322. (e) Pettinari,
C.; Marchetti, F.; Drozdov, A. In Comprehensive Coordination Chemis-
try II; Lever, A. B. P., Ed.; Elsevier Pergamon: Oxford, U.K., 2004; Vol. 1,
pp 211-251. (f) Hierso, J. C.; Amardeil, R.; Bentabet, E.; Broussier, R.;
Gautheron, B.; Meunier, P.; Kalck, P. Coord. Chem. Rev. 2003, 236,
143–206.
(2) In recent years, silyl ligands tethered with two donor groups
(pincer-type ligands) have been extensively studied; see: (a) Balakrishna,
M. S.; Chandrasekaran, P.; George, P. P. Coord. Chem. Rev. 2003, 241,
87–117. (b) Brost, R. D.; Bruce, G. C.; Joslin, F. L.; Stobart, S. R.
Organometallics 1997, 16, 5669–5680. (c) Sangtrirutnugul, P.; Stradiotto,
M.; Tilley, T. D. Organometallics 2006, 25, 1607–1617. (d) MacInnis, M. C.;
MacLean, D. F.; Lundgren, R. J.; McDonald, R.; Turculet, L. Organome-
tallics 2007, 26, 6522–6525. (e) Takaya, J.; Iwasawa, N. J. Am. Chem. Soc.
2008, 130, 15254–15255.
1
The structure of 3 was determined by H, 13C, and 29Si
NMR spectroscopy, elemental analysis, and X-ray structural
analysis. The crystal structure of 3 showed that the three
sulfur atoms were situated on the same side as the H-Si bond
at the silicon atom, although the lone pairs of the sulfur
atoms were not oriented to the center (Figure 1). This pre-
organized structure suggests that 3 can behave as a tetra-
dentate ligand.
(3) Joslin, F. L.; Stobart, S. R. J. Chem. Soc. Chem. Commun. 1989,
504–505.
(4) Gossage, R. A.; McLennan, G. D.; Stobart, S. R. Inorg. Chem.
1996, 35, 1729–1732.
(5) (a) Whited, M. T.; Mankad, N. P.; Lee, Y. H.; Oblad, P. F.; Peters,
J. C. Inorg. Chem. 2009, 48, 2507–2517. (b) Mankad, N. P.; Whited, M. T.;
Peters, J. C. Angew. Chem., Int. Ed. 2007, 46, 5768–5771.
(6) Yan, Y. Y.; RajanBabu, T. V. J. Org. Chem. 2000, 65, 900–906.
r
2010 American Chemical Society
Published on Web 06/08/2010
pubs.acs.org/Organometallics

2840 Organometallics, Vol. 29, No. 13, 2010
Takeda et al.
Figure 1. ORTEP drawing for one fragment of two noniden-
tical silanes 3 in the unit cell with 50% probability thermal
ellipsoids. The other fragment has been omitted, due to the small
differences between the two fragments. All H atoms, except for
the hydrogen connected to Si, have been omitted for clarity.
Figure 2. ORTEP drawing of the iridium(III) complex 4 with
thermal ellipsoids (50% probability). All H atoms, except for the
hydrogen connected to Ir, have been omitted for clarity. Selected
˚
bond lengths (A) and angles (deg): Ir1-Si1, 2.3240(15); Ir1-S1,
Silane 3 reacted with [IrCl(cod)]2 in dichloromethane at
25 °C for 5 h to give the corresponding six-coordinate
iridium(III) complex 4 via oxidative addition of the Si-H
bond to the iridium(I) metal (Scheme 2). A similar reaction
has been reported in the treatment of [2-(Ph2P)C6H4CH2]3-
SiH with [IrCl(cod)]2, giving the corresponding complex
[IrHCl{Si[2-(Ph2P)C6H4CH2]3}].4 The structure of 4 was
2.3439(13); Ir1-S2, 2.4693(15); Ir1-S3, 2.3383(14); Ir1-Cl1,
2.5278(15); Si1-Ir1-S1, 90.92(5); Si1-Ir1-S2, 95.00(5); Si1-
Ir1-S3, 86.61(5); Si1-Ir1-Cl1, 175.24(5); S1-Ir1-S2, 99.83(5);
S2-Ir1-S3, 83.48(5); S1-Ir1-S3, 176.05(6); S1-Ir1-Cl1,
84.32(5); S2-Ir1-Cl1, 85.71(6); S3-Ir1-Cl1, 98.15(6).
The 1H NMR spectrum of 4 showed the H-Ir signal at a
characteristic higher field (-19.66 ppm). In addition, the 1H
NMR spectrum at 60 °C showed the tert-butyl protons as
two singlet peaks at 1.44 and 1.45 ppm with an integral ratio
of 9:18 and the CH2 protons as one sharp singlet at 4.07 ppm
and two doublets at 4.17 and 4.46 ppm with an integral ratio
of 2:2:2. These doublets have geminal coupling constants
1
determined by H and 13C NMR spectroscopy, elemental
analysis, and X-ray structural analysis.
Scheme 2. Synthesis of the Iridium(III) and Platinum(II) Com-
plexes 4 and 5
2
of JHH = 12.6 Hz. These results strongly suggest the six-
coordinate octahedral structure of 4 in solution as well as in
the crystalline state and the rapid inversion on the sulfur
atoms at 60 °C. The 13C NMR spectrum showed two sets of
peaks for the tert-butyl groups, which also supports the six-
coordinate octahedral structure of 4 in solution.
Reaction of 3 with [PtCl2(cod)] in the presence of Et3N
resulted in the formation of the four-coordinate platinum
complex 5 via dissociation of HCl (Scheme 2). Tilley and a
co-worker have reported that treatment of bis(8-quinolyl)-
methylsilane with [PtCl2(cod)] in the presence of Et3N leads
to a similar reaction to give [PtCl{SiMe(8-quinolyl)2}].8
1
The structure of 5 was determined by H and 13C NMR
spectroscopy, elemental analysis, and X-ray structural analysis.
Figure 3 shows the square-planar structure of platinum(II)
complex 5, in which the tripodal silyl ligand acts as a
The structural analysis of 4 revealed the six-coordinate
octahedral structure of 4, in which the silyl group was
situated in a position trans to the chloro ligand (Figure 2).
˚
tridentate ligand. The Pt1-Cl1 bond length (2.455(2) A) is
˚
˚
The Ir-Cl bond length (2.5278(15) A) is close to the reported
slightly longer than those (trans to phenyl) (2.403-2.420 A)
˚
of trans-[PtCl(Ph)(SMe2)2],9 which suggests a slightly larger
or similar trans influence of the silyl group compared with
that of the phenyl group. The long Pt1-S3 distance (5.047(2)
Ir-Cl bond lengths trans to a hydrido ligand (2.50-2.52 A),
and is much longer than the reported Ir-Cl bond lengths
˚
˚
trans to a chloro (2.36-2.40 A) or phosphine (2.42-2.45 A)
ligand.7 This suggests that the silyl group has a trans in-
fluence almost similar to that of hydrido ligands. The Ir-S2
˚
A) indicates no interaction between Pt1 and S3 atoms.
The 1H and 13C NMR spectra of 5 showed that the three
˚
distance (2.4693(15) A) is longer than the other two Ir-S
tert-butyl groups are equivalent at 25 °C, which suggests that
˚
distances (2.3383(14) and 2.3439(13) A), probably due to the
greater trans influence of the hydrido ligand.
(8) Sangtrirutnugul, P.; Tilley, T. D. Organometallics 2008, 27, 2223–
2230.
(9) Kapoor, P.; Kukushkin, V. Y.; Lovqvist, K.; Oskarsson, A.
(7) Robertson, G. B.; Tucker, P. A. J. Am. Chem. Soc. 1982, 104, 317–318.
J. Organomet. Chem. 1996, 517, 71–79.

Communication
Organometallics, Vol. 29, No. 13, 2010 2841
Scheme 3. Reaction of Silane 3 with [PdCl2(PhCN)2]
Figure 3. ORTEP drawing for one fragment of two noniden-
tical platinum complexes 5 in the unit cell with 50% probability
thermal ellipsoids. The other fragment has been omitted, due to
the small differences between the two fragments. All H atoms
and the fragment of solvated hexane have been omitted for
Figure 4. ORTEP drawing of palladium complex 6 with 50%
probability thermal ellipsoids. All H atoms have been omitted
for clarity. Selected bond lengths (A) and angles (deg): Pd1-C1,
2.000(3); Pd1-S1, 2.2646(7); Pd1-Cl1, 2.3691(7); Pd1-Cl1*,
2.4513(7); C1-Pd1-S1, 85.78(8); C1-Pd1-Cl1, 96.10(8); S1-
Pd1-Cl1*, 93.57(2); Cl1-Pd1-Cl1*, 84.57(2).
˚
clarity. Selected bond lengths (A) and angles (deg): Pt1-Si1,
˚
2.326(3); Pt1-S1, 2.308(2); Pt1-S2, 2.304(2); Pt1-Cl1, 2.455(2);
Si1-Pt1-S1, 91.59(9); Si1-Pt1-S2, 87.43(9); S1-Pt1-Cl1,
81.04(8); S2-Pt1-Cl1, 99.85(8).
the three tert-butylthio moieties rapidly exchange among
each other. The rapid exchange of thioether ligands on the
platinum(II) atom at 25 °C has been reported in [PtCl2{P(2-i-
PrSC6H4)3}],10 although it has been reported that the 1H NMR
spectrum of [PtCl2{PPh[2-MeSC6H4]2}] at 10 °C shows a slower
pyramidal inversion of the sulfur atoms and slower exchange
between coordinated and uncoordinated sulfur atoms.11 This
difference may be due to the tert-butyl and isopropyl groups on
the sulfur atoms being bulkier than the methyl groups, which
leads to weaker coordination of sulfur to the platinum.
The NMR studies of 5 revealed rapid exchange among
coordinated and uncoordinated thioether moieties in solu-
tion. Due to these properties, these complexes have potential
for application to catalysts, activation of small molecules,
and stabilization of reactive species. In addition, we found
the Si-C cleavage in the reaction of silane 3 with [PdCl2-
(PhCN)2]. This Si-C cleavage is applicable to the develop-
ment of new reactions for the synthesis of organic and
organosilicon compounds.
Interestingly, silane 3 underwent Si-C bond cleavage by
treatment with [PdCl2(PhCN)2] in the presence of Et3N at
25 °C for 5 days to give the binuclear palladium(II) complex 6 in
63% yield (Scheme 3). The structure of 6 was determined by 1H
and 13C NMR spectroscopy and X-ray structural analysis
(Figure 4). This Si-C cleavage is very interesting, since the Si-C
bonds are relatively stable and the cleavage of an Si-C-
(aromatic) bond by the use of palladium complexes is very rare.12
In this paper, we have synthesized a new silyl ligand
tethered with three thioether moieties and its iridium(III)
and platinum(II) complexes (4 and 5). The X-ray structural
analyses of 4 and 5 indicated the octahedral structure of 4
and square-planar structure of 5. The relatively longer M-Cl
(M=Ir in 4, M=Pt in 5) bond lengths indicate the strong
trans influence of the silyl group, which suggests the ready
cleavage of the M-Cl bonds situated trans to the silyl group.
The elucidation of the reactivities of complexes 4 and 5 and
the mechanism in the reaction of silane 3 with [PdCl2(PhCN)2]
is in progress.
Acknowledgment. This work was partially supported
by a Grant-in-Aid for Scientific Research (C) from the
Japan Society for the Promotion of Science (JSPS). We are
grateful to Professor Soichiro Kyushin, Graduate School
ofEngineering, Gunma University, for his kind permission
to use his single-crystal X-ray diffractometer. In addition,
we thank Associate Professor Shin-ichi Kondo, Depart-
ment of Science, Yamagata University, and Assistant
Professor Masaki Yamamura, Department of Chemistry,
Tsukuba University, for valuable discussions.
Note Added after ASAP Publication. In the version of this
paper published on the Web on June 8, 2010, 29Si NMR
characterization was incorrectly noted for compounds 4 and
5. References to 29Si NMR for these compounds have been
deleted in the version that appears as of July 6, 2010.
(10) Takeda, N.; Tanaka, Y.; Sakakibara, F.; Unno, M. Bull. Chem.
Soc. Jpn. 2010, 83, 157–164.
(11) Abel, E. W.; Dormer, J. C.; Ellis, D.; Orrell, K. G.; Sik, V.;
Hursthouse, M. B.; Mazid, M. A. J. Chem. Soc., Dalton Trans. 1992,
1073–1080.
(12) (a) Valk, J. M.; Boersma, J.; van Koten, G. J. Organomet. Chem.
1994, 483, 213–216. (b) Steenwinkel, P.; James, S. L.; Grove, D. M.;
Kooijman, H.; Spek, A. L.; van Koten, G. Organometallics 1997, 16, 513–
515. (c) Kleij, A. W.; Kleijn, H.; Jastrzebski, J.; Spek, A. L.; van Koten, G.
Organometallics 1999, 18, 277–285.
Supporting Information Available: Text and tables giving
experimental details and characterization data for all new com-
pounds and CIF files giving crystallographic data for compounds
3-6. This material is available free of charge via the Internet at
http://pubs.acs.org.